Conventionally, a bulb lamp has been used as a light source for a vehicle or the like. Recent years, however, luminance intensity of LEDs has become high level so that LEDs are used for a headlight, a winker or the like. In addition, LEDs have become commonly used for a traffic signal, too.
On the other hand, when LEDs are used as a light source, it should be noted that the light emission characteristic thereof is different from that of a conventional bulb lamp. Therefore, the control timing has been studied also in the case where the LED and the bulb lamp are mixed as a light source.
Further, it has been also studied to change the duty ratio and period in accordance with an input signal so as to supply the LED with current corresponding to a pulse width modulated pulse signal. Thus, the luminance change characteristic of the LED becomes nonlinear, so as to be similar to the luminance change characteristic of the bulb lamp.    Patent Document 1: JP-A-7-164958    Patent Document 2: JP-A-2004-235498    Patent Document 3: JP-A-2007-106191